Three naturally ventilated and two mechanically ventilated urban vehicular tunnels were investigated using TSI7575-X and KIMO-VT200 for continuous and single-point measurements during congested periods.The theoretical model was validated by comparing the air velocities, CO and CO2 concentrations with that of the field measurement.Results showed that piston winds existed in every tunnel but the piston effect in mechanically ventilated tunnels was more apparent compared with that in naturally ventilated tunnels.All temperature as well as CO and CO2 concentrations increased from inlets to outlets.Furthermore,the air velocities depended largely on the vehicle speed(vt)and the opening area ratio(Rf).The maximum CO concentrations increased with the decrease in vt and the increase in tunnel length(L)but were minimally affected by Rf.The safety standards for CO concentration depended on exposure time.The allowed tunnel length could be up to 3 000 m at 20 km/h and 1 700 veh/h.